Heres a little idea I picked up many years ago (Some may know where), and have used countless times. It is one of those really simple ideas that just works. You need a loop of strong cord, para cord would be a good start. In short, loop it throught a weapon slot, sling loop, whatever then through itself. You can attach a spring clip straight to it, if you pass the sling throught it and then onto the gun as in my picture, which has a quick release catch at the front, you have a sling that is easy and quick to remove and replace.